1. Field of the Invention
The present invention relates to a method, system, and program for determining the magnetic center shift, or non-repeatable runout errors (NRRO), due to certain magnetic effects, absent mechanical noise.
2. Description of the Related Art
Computer hard disk drives include one or more disks of magnetic storage medium and a disk drive head assembly to read and write data on the magnetic storage medium. Magnoresistive (MR) heads typically include a write element comprised of a thin film inductive head and a read element comprised of a sensor. Manufacturers often test disk drives to determine whether error rates within the disk drive exceed a threshold of acceptability and, therefore, require rejection of the particular disk drive unit. Errors in the ability of an MR head to properly read data from the storage surface can be caused, in part, by track misregistration due to head positioning irregularities on a rotating disk, track interference, and head (magnetic) domain noises. Other causes of errors in disk drives are described in xe2x80x9cMagnetic Disk Drive Technology: Heads, Media, Channel, Interfaces, and Integration,xe2x80x9d by Kanu G. Ashar (1997), which publication is incorporated herein by reference in its entirety.
In determining whether to accept or reject a disk drive, it is desirable to measure the magnetic center shift (xe2x80x9cMCSxe2x80x9d) in the read head. MCS refers to the phenomena of instabilities in the read sensor that cause the read sensor to read an asymmetrical signal. An asymmetrical signal refers to the situation where the read sensor mistakenly reads the position as being more to the left or right of the track center than the actual position. The magnetic fields from the write sensor affect the resistivity of the MR read head thin film, which in turn results in the read sensor reading an asymmetrical position signal. Typically, a bias is applied to the MR head to attempt to compensate for asymmetries and obtain as much of a symmetric signal as possible, i.e., removing the tendency to improperly read a signal too far to the left or right of the track center. Notwithstanding attempts to bias the head to make the signal more symmetric, the MCS noise caused by the magnetic fields of the write head still cause the read sensor to deviate from the center of the track being read. Especially problematic is that MCS affects the ability of the read sensor to measure any signal accurately. Thus, MCS will produce feed forward errors when the read TMR is used to determine a corrective error signal in the closed loop servo system used in disk drives because the signal used in determining the read TMR would include asymmetries. There is thus a need to measure for MCS noise which may indicate structural defects in the MR head and produce additional undetected TMR.
Because the MCS effect is usually indicated by magnetic disturbances, it is in principal possible to measure the extent of MCS by measuring the TMR under presence and non-presence of a magnetic field. Because the magnetic field of concern is usually introduced by the write head, a conventional method compares the track misregistration (TMR) with the write head on with the TMR with the write head off. TMR occurs when the center of the read head is not positioned in the middle of the track during track following. The greater the read head is off-center, the greater the likelihood of errors and noise in reading and writing data from the storage surface of the disk. This increase in noise reduces the signal to noise ratio and the overall quality of data operations. TMR is measured by determining the probability of the read and write heads being a distance from the center of the track. The TMR is the standard deviation from the center or range around the center of the track at which the read head will be offset from the written track center most of the time. To measure the magnetic center shift, the TMR value for the read sensor is measured twice, once with the write head on and again with the write head off. The MCS is defined as the square root of the difference of the respective TMRs squared, as set forth in equation (1) below:                     MCS        =                                            TMR              Write              2                        -                          TMR              Read              2                                                          (        1        )            
One problem with current techniques for measuring MCS, including the method of equation (1), is that a substantial amount of the measured TMR used to estimate MCS includes mechanical noise, such as noise resulting from thermal effects, spindle bearing runouts, mechanical vibrations, and servo loop electronic noise. Mechanical noise as defined herein does not include structural defects in the MR head or head instability. Thus, any measurement of MCS that is based on the TMR, and other current techniques, is often comprised substantially of mechanical noise unrelated to defects with the MR head. More accurate techniques for measuring the MCS resulting primarily from head instability are performed with precision testing equipment, which is typically expensive and requires significant set-up operations. However, such measurement apparatuses are used in the design process and cannot be used to test assembled disk drives. Although the TMR measurement technique may be performed on an assembled disk drive to estimate MCS, such TMR techniques often incorporate substantial mechanical noise. Further, an alterative approach which would involve enlarging the test scope and time to reduce the measurement noise would lead to increased manufacturing cost, which are undesired and impractical.
There, is thus a need to provide an expedient and reliable measurement of MCS on an assembled disk drive that is not dominated by measurements of mechanical noise to allow for a more accurate determination of whether to accept or reject a disk drive based on magnetic center shift.
To overcome the limitations in the prior art described above, preferred embodiments disclose a system, method, and program for determining a value for non-mechanical noise within a disk drive system. The non-mechanical noise is likely related to instability in a head which reads data from a magnetic surface within the disk drive system. First, a sample of position error signals (PES) indicating non-repeatable runouts (NRRO) is provided from read operations within the disk drive system. Spectral analysis is then performed on the provided samples to calculate non-filtered power values for the NRRO values at different frequencies. A filtered power spectrum is determined within a frequency range excluding mechanical noise components. A filtered power value is calculated from the determined filtered power spectrum within the frequency range excluding mechanical noise.
The calculated filtered power value may be compared against a predetermined value to determine whether to reject the disk drive system on the grounds that there is too much non-mechanical noise or noise related to external magnetic fields.
In further embodiments, the spectral analysis comprises a fourier analysis. In still further embodiments, the filtered power value is determined by first calculating the inverse of the fourier coefficients to determine the NRRO values that do not include mechanical noise. Second, the filtered power value is calculated by summing the product of the inverse of the fourier coefficients and the fourier coefficients between a range of values that exclude mechanical noise.
Preferred embodiments provide a method for estimating the magnetic center shift component of the NRRO signal, which is due to magnetic affects, particularly from the write head and structural defects in the thin film of the MR head. A power value is then calculated for this magnetic center shift using a discrete fourier transform. If the calculated power of the noise exceeds a predetermined threshold, then the disk file is rejected as having too much MCS noise. Preferred embodiments provide a method for determining magnetic center shift, i.e., MCS noise absent mechanical effects, after the disk drive has been assembled, using electronics within the disk drive and/or an external computer to which the disk drive is connected. After estimating the magnetic center shift for the disk drive, a determination can then be made on whether to reject or accept the tested disk drive.